Products molded from a resin material generally have excellent electrical insulation properties and, hence, are widely used in the field of electric and electronic components. However, elements of electric and electronic components molded from a resin material alone are liable to be electrostatically charged because of their high electrical insulation properties, thereby presenting such a problem that electronic components such as integrated circuits may be damaged due to adhesion of dust or electric discharge. Therefore, resin molded products for use in the field of semiconductor device production are typically imparted with a slight electrical conductivity by various methods.
One of the simplest methods for imparting the electrical conductivity to a resin molded product is to apply a solution of a surface active agent onto the resin molded product. In accordance with this method, however, a resin molded product produced in one step is subjected to the application of the surface active agent solution in another step, so that the step of applying the surface active agent solution is required in addition to the step of molding the resin material. Further, the electrical conductivity of the resin molded product obtained through this method is susceptible to the ambient humidity. That is, the resin molded product is likely to exhibit an intended level of electrical conductivity with its surface being kept in a readily humidified state (under highly humid conditions), but tends to fail to exhibit a required level of electrical conductivity with its surface being kept in a hardly humidified state (under dry conditions). Moreover, the surface active agent applied onto the resin molded product by this method is often absorbed into the molded product or removed therefrom by abrasion, so that reduction in the electrical conductivity with time is inevitable. Therefore, it is difficult for the resin molded product to retain the electrical conductivity for a long time. In addition, the removed surface active agent may cause contamination in a semiconductor device production process.
Instead of the aforesaid post-treatment of the resin molded product by the application of the surface active agent solution, an attempt has recently been made to initially impart the electrical conductivity to a resin molded product per se. More specifically, a conductivity imparting material is preliminarily blended in a resin material and the resulting resin material is molded into a desired shape for production of a conductive resin molded product. Generally used as the conductivity imparting material are antistatic agents such as a surface active agent and electrically conductive fillers such as of metal materials and carbon materials.
Where an antistatic agent such as a surface active agent is selected as the conductivity imparting material, the antistatic agent gradually migrates from the inside to the surface of the resin molded product, so that much time is required before the resin molded product exhibits an electrical conductivity. Since the effectiveness of the antistatic agent differs depending on the type of the resin material to be used, it is necessary to select a suitable antistatic agent for the resin material in consideration of the glass transition temperature and crystallinity of the resin material and the compatibility with the resin material. Further, the antistatic agent which has migrated to the surface of the resin molded product is often removed by abrasion as in the aforesaid solution application method, thereby possibly causing contamination in the semiconductor device production process.
In the case of the electrically conductive filler, on the other hand, the electrical conductivity can immediately be imparted to the resin molded product simply by blending a proper amount of the conductive filler in a resin material. Unlike the antistatic agent, there is no need to consider the combination between the conductive filler and the resin material (i.e., the conductive filler has a versatility with respect to various resin materials), so that the conductive filler can more easily and stably impart the conductivity to the resin molded product than the antistatic agent.
In the meantime, as a resin molded product containing an electrically conductive filler, Japanese Laid-open Patent Publication No. 63-53017 discloses a resin molded product which is obtained by molding a resin composition containing 64 to 80 vol % of a resin and 36 to 20 vol % of an electrically conductive substance, and which has attained a desired resistance by applying a voltage of not more than 1,000V. The electrically conductive substance used herein is a particulate or fibrous good electric conductor such as a metal, a metal oxide or a carbon, or a mixture thereof and, since its specific gravity is supposed to be generally larger than 1, the resin molded product is supposed to contain at least 20 wt % of the electrically conductive substance.
Further, Japanese Laid-open Patent Publication No. 62-110917 discloses an electrically conductive composite linear body, i.e. a resin molded product, in which a process employing a high voltage of not more than 10 kV is performed on a composite linear body obtained by disposing a covering layer of an insulating polymer on a linear body (core body) formed from a polymer (resin material) containing an electrically conductive substance. The electrically conductive substance as used herein is, for example, carbon black, and the amount of its use is, for example, 20 to 200 wt % with respect to the weight of the polymer.
However, the electrically conductive filler is generally more expensive than the resin material. For example, the prices in Japan of polypropylene resin and modified polyphenylene oxide resin, which are widely used for producing a resin molded product, are approximately 100 yen/kg and 1,000 yen/kg, respectively, at around the time of filing the present application, whereas the prices in the same period in Japan of pitch-based carbon fibers and carbon black used as an electrically conductive substance are approximately 3,000 yen/kg and 500 to 1,000 yen/kg, respectively. Therefore, since the resin molded products disclosed in the above-mentioned Publications each contain a large amount of an electrically conductive filler mixed with a resin material, the resin molded products will be extremely expensive though a desired electrical conductivity can be imparted. Particularly, where the resin molded product needs to be imparted with an electrical conductivity comparable to that of a metal for enhancement of electromagnetic shieldability, for example, a great amount of the conductive filler should be added to the resin material, so that the cost of the resin molded product is significantly increased. In this case, the conductive filler contained in a great amount in the resin molded product may deteriorate intrinsic properties of the resin molded product, and is liable to be released from the resin molded product to cause contamination.
Moreover, since the resin molded product disclosed in the above-mentioned Japanese Laid-open Patent Publication No. 63-53017 contains a large amount of the conductive filler, its color tone is strongly influenced by the color of the conductive filler. Particularly, since the resin molded product disclosed in the examples of the Publication contains a large amount of carbon fiber or graphite powder as the conductive filler, the color is inherently black and it is extremely difficult to freely impart a desired color to the resin molded product itself. In the meantime, since the resin molded product disclosed in Japanese Laid-open Patent Publication No. 62-110917 has a covering layer disposed on a core body for achieving a color that harmonizes with another non-conductive fibers, the resin molded product has a two-layer structure made of the core body and the covering layer, thereby providing a complex construction.
It is an object of the present invention to enhance the electrical conductivity of a resin molded product and, particularly, to reduce the surface resistance of the resin molded product, while reducing the amount of an electrically conductive filler to be added to the resin molded product.
It is another object of the present invention to realize a resin molded product exhibiting a high electrical conductivity, particularly a low surface resistance, though the amount of the added electrically conductive filler is reduced, and being imparted with a color.